Capacitors for use in electronic instruments such as cellular telephones and personal computers are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode material. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or use a sintered body increased in the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the specific surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and this makes it difficult to impregnate a cathode agent in the later step. As one of means for solving these problems, a capacitor using a sintered body of a material having a dielectric constant larger than that of tantalum, is being studied. Niobium is known as the material having a larger dielectric constant. Niobium and tantalum belong to the same group and therefore, tantalum is present as an impurity in niobium.
JP-A-2001-307963 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (U.S. Pat. No. 6,540,810) discloses that when a niobium powder is purified to obtain a niobium powder reduced in the tantalum content to 700 mass ppm or less, and a sintered body and then a capacitor are produced using this powder, the CV value or capacitance is not decreased. However, if a purification step is provided in the production process, the production time of niobium powder is prolonged and the recovery decreases.